In mobile communication systems, data service has been gradually increased, and thus, in most cases, TCP (transmission control protocol) is employed as an upper layer protocol. However, since the TCP is designed on the basis of a wired network, the TCP does not reflect wireless channel errors frequently occurring in a wireless network. Therefore, in the TCP, a wireless channel error may be recognized as channel congestion so that congestion control is performed.
On the other hand, it is expected that a next-generation network has an overlay structure, and thus, vertical handover does more frequently occur according to movement of mobile terminals.
However, when a mobile terminal enters from a network having a relatively narrow coverage, a high transmission rate, and a short RTT (round trip time) such as a WLAN (public wireless LAN service) into a network having a wide coverage, a low transmission rate, and a long RTT such as a 3G network, the mobile terminal undergoes handover. When upward vertical handover is executed, although packet loss caused by channel congestion according to a difference in network characteristics such as an RTT does not occur, a retransmission timeout event may occur in a TCP sender.
In the conventional TCP such as a TCP Reno, the retransmission time-out event is recognized as channel congestion, and thus, congestion control is performed when the retransmission time-out event generates, so that a transmission rate is greatly decreased.
Therefore, in the related art, a technique of preventing congestion control from being performed due to erroneous recognition of the occurrence of a retransmission time-out event as channel congestion during the upward vertical handover has been required to be developed.